Refractory products include fused cast products, which are well known in the construction of glass-melting furnaces, and sintered products.
In contrast to sintered products such as those described, for example, in U.S. Pat. No. 4,507,394, fused cast products usually include an intergranular vitreous phase connecting together the crystalline grains. The problems posed by sintered products and by fused cast products, and the technical solutions adopted to overcome them thus generally differ. A composition that has been developed for the production of a sintered product is thus not, a priori, of use per se in producing a fused cast product, and vice versa.
Fused cast products, often termed electrofused products, are obtained by melting a mixture of appropriate starting materials in an electric arc furnace or by using any other technique that is suitable for such products. The molten liquid is then cast into a mold and the product obtained undergoes a controlled cooling cycle to bring it to ambient temperature without fracturing. This operation is termed “annealing” in the art.
Fused cast products include electrofused products with a high zirconia content, i.e. comprising more than 85% by weight of zirconia ZrO2); (they are well known for their very good resistance to corrosion without coloring the glass produced and without generating defects.
Conventionally, fused cast products with a high zirconia content also include sodium oxide (Na2O) to prevent the formation of zircon from the zirconia and silica present in the product. Zircon formation is in fact deleterious, since it is accompanied by a reduction in volume of the order of 20%, thus creating mechanical stresses that are the source of cracks.
The product ER-1195, produced and marketed by the Société Européenne des Produits Réfractaires and protected by European patent EP-B-0 403 387, is currently widely used in glass-melting furnaces. Its chemical composition comprises approximately 94% of zirconia, 4% to 5% of silica, approximately 1% of alumina, 0.3% of sodium oxide and less than 0.05% by weight of P2O5. It is typical of products with a high zirconia content that are used for glass furnaces.
French patent FR-A-2 701 022 describes fused cast products with a high zirconia content that contain 0.05% to 1.0% by weight of P2O5 and 0.05% to 1.0% by weight of boron oxide, B2O3. Said products have a high electrical resistivity. This can advantageously stabilize the consumption of electricity during electric melting of the glass and above all, this can overcome any problems with short circuiting in refractories, causing their rapid degradation. During electric melting of glass, some of the electric current passes through the refractory products. Thus, increasing the resistivity of said refractory products can reduce the quantity of electric current that passes through them.
International patent document WO-A-2005/068393 describes fused cast products with a high zirconia content having a high electrical resistivity while minimizing the quantities of BaO, SrO, MgO, CaO, P2O5, Na2O, and K2O. Said products contain 0.1% to 1.2% by weight of B2O3.
Japanese patent document JP 2000 302 560 describes fused cast products that do not contain Nb2O5 or Ta2O5.
The current trend for very high quality glass, in particular glass for LCD type flat screens, is increasing the demand for refractory products from glass-melting furnaces. In particular, there is a need for refractory products with further improved electrical resistivity while retaining good feasibility and good service characteristics.